Scientist use the presence of methane as a possible indicator of life.

Lyle Whyte, vice-chair of the Canadian Astrobiology Network, said that it was a bad day for the search for life on Mars. "It's not a definitive sign saying that there is no life on Mars, but it's not a good sign," he told ABC News. "We have a problem."

Whyte, who was not involved with the paper but collaborates with NASA on other projects, said that methane is like a smoking gun for microbial life. "Ninety percent of the methane of Earth's atmosphere originate from microbes," he said. "That's why people got excited about methane in Mars' atmosphere."

The main evidence for methane on Mars comes from Michael Mumma, a planetary scientist at NASA. One of his papers combined methane observations from the Keck Observatory and the Infrared Telescope Facility, both in Hawaii. "When we saw the methane plumes, we saw values range from 20 to 60 parts per billion," he said. "Curiosity's upper limit is only 1.4 parts per billion."

Paul Mahaffy, one of the authors of the new paper, said that Curiosity's instruments are more sensitive than what observatories have to offer. "It's much more sensitive than the best measurement done from Earth's surface," he said. "If there were a strong source of methane on Mars, we would have seen it by now."

Mumma's original observations also say that methane in the atmosphere isn't always there. "The concentration of methane changes with location, the time of day, and even the season on Mars," he said. "It's actually rare when we do see methane."

Curiosity made its methane observations at six different locations along its journey. Though the evidence seems to be pointing to the absence of methane, Mahaffy said that it's still worth doing more measurements. "Every few weeks, we'll sample the atmosphere for methane," he said. "From our point of view, it's still an interesting story."

While Mumma and Mahaffy are eager to determine what the relationship is with the planet and methane, they still say that it's not the deciding factor for determining whether life currently exists on Mars. The gas can also be created geologically, if hot water buried deep beneath the planet's surface reacts with hydrogen gas and carbon dioxide.

"We aren't able to distinguish biological production from geological production," said Mumma. "But that's where we are, and we're still continuing our research today."